1. Field of the Invention
The invention concerns a method for operating a firing plant using fossil fuels by means of a burner. It also concerns the design of such a burner.
2. Discussion of Background
In firing plants for central heating, for example, the fuel--in the conventional type--is sprayed by means of a nozzle into a combustion space and burned there with the addition of combustion air. The operation of such firing plants is fundamentally possible by means of a gaseous or liquid fuel. When using a liquid fuel, the weak point with respect to clean combustion in terms of NO.sub.x, CO and UHC emissions is mainly associated with the complete atomization of the fuel necessary and its mixture with the combustion air.
When a gaseous fuel is used, the combustion is characterized by a substantial reduction in terms of pollutant emissions because the gasification of the fuel--in contrast to a liquid fuel--is already provided. Particularly in the case of firing plants for domestic use, however, gas-operated burners have not become generally accepted, despite the many relevant advantages which such firing plants can offer. The reason for this may be that the procurement of gaseous fuels requires an expensive infrastructure, also that these gasification burners require much maintenance and finally, last but not least, that undesirable deposits can form in the gasification chamber during operation. These very rapidly impair the effectiveness of the gasification substantially, leading to deterioration of the combustion and to an increase in the pollutant emissions. If no gaseous fuel is available, the firing plant goes out of action.
If, in consequence, a liquid fuel is used, the quality of the combustion in terms of low pollutant emissions depends substantially on whether an optimum degree of mixing of the fuel/fresh air mixture has been successfully prepared, i.e. whether complete gasification of the liquid fuel has been ensured. The problems of operation in the part load range, as dealt with in EP-A2-0 166 329, should also be noted. Various other attempts which have become known to operate with a leaner mixture at part load have invariably failed because in these cases, the burn-out deteriorates and the CO/UHC emissions increase greatly. In the specialist language, this condition is described under the designation of CO/UHC-NO.sub.x dilemma. Providing a premixing zone for the fuel/fresh air mixture before the actual combustion zone does not lead to the objective of an operationally reliable burner either because in this case, there is the imminent danger that a flashback from the combustion zone into the premixing zone could damage the burner.
A burner is known from EP-A1-0 210 462 which is satisfactory for the combustion of liquid and, if need be, gaseous fuel. This burner manages without a premixing zone. The spraying in of the liquid fuel and its preparation for mixing with the added air towards gasification condition occurs in such a way that the generation of premixing becomes superfluous. Since, with this subject-matter, the momentum with which the liquid fuel is sprayed in is matched to the load of the plant, the mixture is never too lean or too rich. Although the advantages of the burner considered here in terms of pollutant emissions cannot be denied, it has been found that the NO.sub.x and CO emission values, although they are lower than the legal limiting values, will have to be further substantially reduced in future. It has also been found that coking problems, arising from the way in which the liquid fuel is sprayed in, cannot be excluded and that the fuel spraying system itself is not easy to deal with. A further problem is provided by the excess air and its temperature in the course of the mixture formation in this burner, which does not ensure complete evaporation and hence gasification of the liquid fuel before ignition of the mixture. It has also been found that the use of such a burner in atmospheric-pressure firing plants does not provide optimum operation because, even if the degree of gasification of the liquid fuel were substantially achieved, there would be no effect on the locally high flame temperature peaks which, as is known, are responsible for the formation of NO.sub.x.